Datasheet ADT7420 (Analog Devices) - 10

ADT7420 Data Sheet THEORY OF OPERATION CIRCUIT INFORMATION NORMAL MODE
The ADT7420 is a high accuracy digital temperature sensor that In normal mode (default power-up mode) the ADT7420 runs uses a 16-bit ADC to monitor and digitize the temperature to an automatic conversion sequence. During this automatic con- 0.0078°C of resolution. The ADC resolution, by default, is set to version sequence, a conversion typically takes 240 ms to complete 13 bits (0.0625°C). An internal temperature sensor generates a and the ADT7420 is continuously converting. This means that voltage proportional to absolute temperature, which is com- as soon as one temperature conversion is completed, another pared to an internal voltage reference and input into a precision temperature conversion begins. Each temperature conversion digital modulator. result is stored in the temperature value registers and is available The internal temperature sensor has high accuracy and linearity through the I2C interface. In continuous conversion mode, the over the entire rated temperature range without needing correc- read operation provides the most recent converted result. tion or calibration by the user. On power-up, the first conversion is a fast conversion, taking The sensor output is digitized by a sigma-delta (Σ-Δ) modulator, typically 6 ms. If the temperature exceeds 147°C, the CT pin also known as the charge balance type analog-to-digital conver- asserts low. If the temperature exceeds 64°C, the INT pin asserts ter. This type of converter utilizes time-domain oversampling low. Fast conversion temperature accuracy is typically within ±5°C. and a high accuracy comparator to deliver 16 bits of resolution The conversion clock for the part is generated internally. No in an extremely compact circuit. external clock is required except when reading from and
CONVERTER DETAILS
writing to the serial port. The Σ-Δ modulator consists of an input sampler, a summing The measured temperature value is compared with a critical network, an integrator, a comparator, and a 1-bit DAC. This temperature limit (stored in the 16-bit TCRIT setpoint read/write architecture creates a negative feedback loop and minimizes the register), a high temperature limit (stored in the 16-bit THIGH set- integrator output by changing the duty cycle of the comparator point read/write register), and a low temperature limit (stored in output in response to input voltage changes. The comparator the 16-bit TLOW setpoint read/write register). If the measured samples the output of the integrator at a much higher rate than value exceeds these limits, the INT pin is activated; and if it exceeds the input sampling frequency. This oversampling spreads the the TCRIT limit, the CT pin is activated. The INT and CT pins are quantization noise over a much wider band than that of the programmable for polarity via the configuration register, and the input signal, improving overall noise performance and INT and CT pins are also programmable for interrupt mode via increasing accuracy. the configuration register.
ONE-SHOT MODE Σ-∆ MODULATOR
Setting Bit 6 to 0 and Bit 5 to 1 of the configuration register
INTEGRATOR COMPARATOR
(Register Address 0x03) enables the one-shot mode. When this
VOLTAGE REF
mode is enabled, the ADT7420 immediately completes a
AND VPTAT
conversion and then goes into shutdown mode. Wait for a minimum of 240 ms after writing to the operation
1-BIT
mode bits before reading back the temperature from the tem-
DAC
perature value register. This time ensures that the ADT7420 has time to power up and complete a conversion.
1-BIT CLOCK LPF DIGITAL TEMPERATURE
To obtain an updated temperature conversion, reset Bit 6 to 0 12
GENERATOR FILTER VALUE
0
13-BIT/
3- and Bit 5 to 1 in the configuration register (0x03).
REGISTER 16-BIT
01 09 Figure 11. Σ-Δ Modulator The one-shot mode is useful when one of the circuit design priorities is to reduce power consumption. The ADT7420 can be configured to operate in any one of the following four operating modes: normal, one-shot, 1 SPS, and shutdown. Rev. A | Page 10 of 24 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PRODUCT HIGHLIGHTS FUNCTIONAL BLOCK DIAGRAM TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS I2C TIMING SPECIFICATIONS Timing Diagram ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION CIRCUIT INFORMATION CONVERTER DETAILS NORMAL MODE ONE-SHOT MODE CT and INT Operation in One-Shot Mode 1 SPS MODE SHUTDOWN FAULT QUEUE TEMPERATURE DATA FORMAT TEMPERATURE CONVERSION FORMULAS 16-Bit Temperature Data Format 13-Bit Temperature Data Format 10-Bit Temperature Data Format 9-Bit Temperature Data Format REGISTERS ADDRESS POINTER REGISTER TEMPERATURE VALUE REGISTERS STATUS REGISTER CONFIGURATION REGISTER THIGH SETPOINT REGISTERS TLOW SETPOINT REGISTERS TCRIT SETPOINT REGISTERS THYST SETPOINT REGISTER ID REGISTER SERIAL INTERFACE SERIAL BUS ADDRESS WRITING DATA READING DATA RESET GENERAL CALL INT AND CT OUTPUTS UNDERTEMPERATURE AND OVERTEMPERATURE DETECTION Comparator Mode Interrupt Mode APPLICATIONS INFORMATION THERMAL RESPONSE TIME SUPPLY DECOUPLING POWERING FROM A SWITCHING REGULATOR TEMPERATURE MEASUREMENT QUICK GUIDE TO MEASURING TEMPERATURE OUTLINE DIMENSIONS ORDERING GUIDE